A trim retarder compensates for residual retardation of the micro-display imager panel in a dark (OFF) state. Unlike typical birefringent waveplates providing ¼λ or ½λ retardation, a typical A-plate trim retarder provides between 1 nm and 50 nm of in-plane retardance. The primary benefit of introducing a trim retarder into a display system is to enhance image contrast, while not significantly degrading the ON-state brightness. In conventional LCOS and xLCD display systems, illustrated in FIGS. 1a and 1b, respectively, one or more trim retarders 2 are positioned adjacent to imager panels 3, typically less than an inch diagonal, for receiving a cone of light 4, typically with a cone illumination of ±12°, from a polarization beam splitter 5.
Conventional thermotropic liquid crystals in reflective LCOS imager panels are either twisted nematic, e.g. 45° twist (45TN), or vertically-aligned nematic (VAN-mode), which get switched (or relaxed) to near homeotropic orientation. Other LC-modes in reflective LCOS and transmissive xLCD, i.e. bend-aligned nematic or pi-cell, also require trim retarders, if the LC-technology employs a dark-state director orientation near the homeotropic alignment. A VAN-mode cell on a reflective substrate is functionally equivalent to a pi-cell in transmission mode, i.e. both operate as electrically controllable birefringence for gray-scale with viewing angle symmetry about an axis orthogonal to the LC tilt-plane.
In homeotropic alignment the LC uniaxial positive molecules are oriented normal to the device plane. The dark-state may be a switched state or a relaxed state, depending on LC modes. In most applications, a true homeotropic orientation in the dark state is not suitable, i.e. a pre-tilt is required to provide consistent and faster switching behavior. Moreover, true homeotropic orientation in the dark state may not be available due to a lack of high voltage supplies in 45TN panels, or due to boundary LC layers being anchored by alignment surface effects. As a consequence, the display panels exhibit both an in-plane and an out-of-plane residual retardation component, i.e. A-plate and C-plate components, respectively. Due to the use of positive-only uniaxial LC in LCD panels, the c-plate component is always positive, thereby adding to the net panel retardance at off-axis illumination.
In typical polarization-based light engine architectures, the imager panel illumination lens has a reasonably small focal ratio (f/#), i.e. focal length/iris diameter, in order to provide for adequate light throughput. A typical f/# is 2.5 for an inch diagonal imager panel size and approximately ±12° cone angles in air. Accounting for the imager panel and the light engine characteristics, the residual retardation compensation can be broadly divided into two steps: first the in-plane retardation (IPR) component of the imager panel is negated by aligning an A-plate component with the optic axes of the imager panel (also c-axes) at 90° relative azimuth, and then improving the field of view by removing the imager out-of-plane retardance (OPR) with a negative c-plate (NCP) retarder component. Note that the optic axes may deviate slightly from the nominal crossed orientations due to the mismatch in the IPR of imager panel and the trim retarder.
IPR compensation is almost always the primary step due to the lack of true homeotropic LC orientation. There is quite a mismatch between the relative contribution of the imager IPR and OPR to the overall net retardation under a cone illumination. As an example, the VAN-mode panel is aligned with an 80° to 89° oblique-tilt (typically 80° to 83°). The effective e-wave indices of refraction parallel and orthogonal to the device plane are given by the following expressions obtained from uniaxial index ellipsoid formula:
                                          1                                          n                e                2                            ⁡                              (                                                      θ                    c                                    ;                                      in                    ⁢                                          -                                        ⁢                    plane                                                  )                                              =                                                                      sin                  2                                ⁡                                  (                                      θ                    c                                    )                                                            n                e                2                                      +                                                            cos                  2                                ⁡                                  (                                      θ                    c                                    )                                                            n                o                2                                                    ,        and                            (        1        )                                                      1                                          n                e                2                            ⁡                              (                                                      θ                    c                                    ;                                      out                    ⁢                                          -                                        ⁢                    plane                                                  )                                              =                                                                      sin                  2                                ⁡                                  (                                      θ                    c                                    )                                                            n                o                2                                      +                                                            cos                  2                                ⁡                                  (                                      θ                    c                                    )                                                            n                e                2                                                    ,                            (        2        )            
where ne and no are intrinsic material extraordinary and ordinary indices of refraction at a given wavelength, and θc is the c-axis tilt from the device normal direction. The decomposition of an o-plate, i.e. VAN-mode, positive uniaxial retardation into two orthogonal components is illustrated in FIG. 2. Taking example LC parameters of Δn=0.15 (ne=1.65 and no=1.50) and IPR of 5 nm single pass, OPR values are 180 nm and 380 nm single pass @ θc=80° and 83°, respectively, which are within the range of commercially available VAN-mode panels. For a reflective LCOS imager panel, the overall imager OPR has to include the undesirable off-axis birefringence effects of metallic reflectors at the silicon backplane. For other important LC-modes, namely the 45TN-mode, the range of IPR typically falls within 15 nm to 25 nm, and the associated OPR has about 200 nm within the visible wavelength spectrum. For any imager panel with both IPR and OPR, one could assign the ratio of OPR to IPR asγ=Γc/Γaγ(VAN)=36× to 76× or more depending on pre-tilt, andγ(45TN)=8× to 13×.
With reference to FIG. 3, which illustrates a conventional two-layer A/C-plate retarder model, the IPR component falls off as a function of cos(θo), and the OPR component increases as a function of sin2(θo)cos(θo), where θo is the refractive angle for the o-ray in the birefringent medium. These approximations are valid for a limited cone angle, e.g. up to ±30° in air. For a limited cone angle of ±12°, the roll-off effect on IPR is negligible; however, the net retardation contribution from the OPR is approximately,Γc′=Γc(intrinsic)×η, where η varies from 0 to 1.9% for AOI=0° to 12°.
Assuming the average net retardance contributions from the OPR over 0° to 12° is 1%, then from the average multiples (γ) of intrinsic OPR to IPR values, the contributions of OPR to the effective retardations are given by,Γc′=ηγΓa≈ 1/100*50*Γa, or 0.5Γa for VAN-mode andΓc′=ηγΓa≈ 1/100*10*Γa, 0.1Γa for 45TN-mode.
The effect of OPR at angles of incidence of less than 12° is only a fraction of the corresponding IPR retardation. These typical retardation components for VAN and 45TN-mode illustrate the critical need to first compensate for the IPR, and then improve the field of view properties of the display. It also suggests that it is paramount to compensate for the OPR in a VAN-mode panel, whereas a 45TN-mode panel has much less to gain from NCP compensation.
An in-plane retardation component can be fabricated using the LCP/LPP technology, configured as an A-plate (planar LC director alignment) or an O-plate (oblique LC director alignment), as disclosed in copending patent application 60/529,315 filed Dec. 11, 2003; 60/587,924 filed Jul. 14, 2004; and 60/589,167 filed Jul. 19, 2004, which are incorporated herein by reference.
In order to introduce a negative C-plate component, with the c-axis of the LC medium perpendicular to the device plane, an averaging effect of tight-pitch cholesteric LC has been proposed. For the cholesteric negative c-plate to work, the helical pitch LC medium must be shorter than the shortest wavelength in the visible wavelength range, i.e. a pitch value of 250 nm. Unfortunately, the use of cholesterics with in-plane LC director alignment may present unwanted crossed polarization leakage into an LCOS projection system, due to the high intrinsic material birefringence involved.
Alternatively, in the area of crystal waveplates, a pseudo-zero order waveplate retarder can be fabricated by crossing optic axes of two birefringent plates. The individual layers may be positive, e.g. single-crystal quartz, or negative, e.g. Calcite, birefringence. This arrangement can also used for fabricating achromatic waveplates utilizing two waveplate elements with appropriate dispersion profiles, i.e. single-crystal quartz and magnesium fluoride combinations.
U.S. Pat. No. 5,196,953 issued to Yeh et al on Mar. 23, 1993 discloses a transmissive LCD device incorporating dielectric form birefringence compensator in which the LCD is compensated by creating the conditions:|ΔnL|dL=|ΔnC|dc,
wherein Δn is the birefringence, d is the layer thickness, and subscripts ‘L’ and ‘C’ refer to the switchable LC-layer and the dielectric form birefringence compensator, respectively. In a preferred embodiment the lower and higher index values of n0 and ne in the LC layer and the compensator sections are matched. Unfortunately, this approach greatly restricts the type of dielectric form birefringence compensator material for use therein, and requires accurate measurement of material constants and coating thicknesses. Moreover, this method does not take into account the retardance caused by an off-axis reflection from the air/substrate interface. Furthermore, limiting the no and ne to those of the LC-layer would necessitate very thick coating layers for large −C values.
Conventional antireflection coating designs, such as those disclosed in U.S. Pat. No. 2,478,385 issued Aug. 9, 1949 in the name of Gaiser, U.S. Pat. No. 3,185,020 issue May 25, 1965 in the name of Thelen, and U.S. Pat. No. 3,604,784 issued Sep. 14, 1971 in the name of Louderback et al, are comprised of three layers, which cause destructive interference between reflected and refracted light within a given wavelength band defined by a center wavelength. The first layer has an optical thickness of a quarter wavelength of the center wavelength and a low refractive index, the second layer has an optical thickness of a half wavelength of the center wavelength and a high refractive index, and a third layer has an optical thickness of a quarter wavelength of the center wavelength and a medium refractive index, together forming a QHQ AR structure.
Further advancements in antireflection coatings, which are disclosed in U.S. Pat. No. 3,463,574 issued Aug. 26, 1969 to Bastien et al, U.S. Pat. No. 3,565,509 issued Feb. 23, 1971 to Sulzbach, U.S. Pat. No. 3,781,090 issued Dec. 25, 1973 to Sumita, U.S. Pat. No. 3,799,653 issued Mar. 26, 1974 to Ikeda, U.S. Pat. No. 3,936,136 issued Feb. 3, 1976 to Ikeda et al, U.S. Pat. No. 4,313,647 issued Feb. 2, 1982 to Takazawa, and U.S. Pat. No. 4,666,250 issued May 19, 1987 to Southwell et al relate to multi-layer antireflection coatings and the use of Herpin equivalents to design multi-layer structures with the desired index of refraction.
An object of the present invention is to overcome the shortcomings of the prior art by providing a predictable and environmentally stable optical trim retarder for both in-plane and out-of-plane residual retardation components in transmissive, e.g. xLCD, and reflective, e.g. LCoS, image panels.